Strontium niobate electro-optic modulator

ABSTRACT

A single domain crystal of strontium niobate exhibits a substantial transverse electro-optic effect. Intensity, frequency and phase modulators for laser beams, or the like either continuous or pulse, utilizing the strontium niobate crystal may be operated over a broad bandwidth including the visible spectrum at low modulation voltage and power.

United States Patent 1191 Nanamatsu et al.

[111- 3,747,022 1451 July 17,1973

[ STRONTIUM NIOBATE ELECTRO-OPTIC MODULATOR [7 5] Inventors: SatoshiNanamatsu; Masakazu Kimura; Kikuo Doi, all of Tokyo, Japan [73]Assignee: Nippon Electric Company, Limited,

Tokyo, Japan 22 Filed: Mar. 19, 1971 21 Appl. No.2 126,229

[30] Foreign Application Priority Data March 20, 1970 Japan ..45/243l6[52] US. Cl 332/751, 350/160, 252/300 [51] Int. Cl. H015 3/02 [58] Fieldof Search 332/7.5l; 350/150,

[56] References Cited OTHER PUBLICATIONS lsupou, Dielectric Constant ofNiobates & Tantalates Primary Examiner-Benjamin A. Borchelt AssistantExaminer-N. Moskowi'tz Attorney-Sandoe, Hopgood & Calimafde [57]ABSTRACT A single domain crystal of strontium niobate exhibits asubstantial transverse electro-optic effect. Intensity, frequency andphase modulators for laser beams, or the like either continuous orpulse, utilizing the strontium niobate crystal may be operated over abroad bandwidth including the visible spectrum at low modulation voltageand power.

4 Claims, 2 Drawing Figures Volloge S ource I7 1 2 Modulated Laser '3 9Beam Sr2Nb2O Crystal PATENTEB JUL 1 7 I973 SF2Nb207 Crystal INVENTORSSATOSHI NIANAMATSU MASAKAZU mMURA KIKUO D01 v ATTORNEYS BACKGROUND OFTHE INVENTION This invention relates generally to modulators forelectromagnetic wave at frequencies including those of the visiblespectrum, and more particularly to modulators for coherent light beamsof the type operating on the electro-optic principle. Electro-opticmodulators depend for their operation upon a single crystal materialwhich modifies a transmission property of a transmitted electromagneticwave by the application of an electric field. Such modulators may becontinuous or pulse and may result in a variation in the intensity,frequency or phase of the transmitted electromagnetic wave.

Potassium dihydrogen phosphate (KDP) is the material most commonlyemployed in electro-optic modulators, at the present time. The crystalof KDP is, however, inadequate to practical usage because of itsdeliquescence and brittleness, the latter quality causing difficultiesin cutting and polishing the crystal.

Lithium niobate (LiNb single crystal has been recently found to be aphysically and chemically stable material for use in light modulators.This crystal can be readily grown in a crystal size necessary formodulators by the Czochralski method and can be readily cut andpolished. The electro-optic effect of this crystal is greater than thatof KDP. However,this crystal has a major deficiency in that irradiationof the crystal by the electromagnetic wave causes a change in therefractive index of the irradiated part of the crystal causing asocalled radiation damage.

Lithium .tantalate (LiTaO has been found to be a suitable material foruse as an optical modulator which shows no evidence of radiation damageand has all of the excellent physical and chemical stability andelectro-optic activity of LiNbO However, the modulating properties ofLiTaO is insufficient for use in broadband, highly sensitive modulators.

SUMMARY OF THE INVENTION In accordance with the invention, single domaincrystals of an approximate nominal composition Sr Nb o have been foundto exhibit a greater electro-optic effect than that of LiTaO Like LiTaOSr Nb O manifests no radiation damage when irradiated by electromagneticwave over long periods, has excellent physical and chemical stability,and can be readily grown in large crystal by conventional techniquessuch as the Czochralski method and floating zone technique to yield auniform single crystal of good quality.

Electro-optic modulators of the invention utilizing Sr Nb O, singlecrystals can operate either continuous or pulse and result in eitherintensity, frequency, or phase variation of the electromagnetic wavewith broad bandwidths. A particular advantage of the invention residesin that the modulator operates with high efficiency at a sufficientlylow modulating voltage, so that the modulating circuit may betransistorized and the over-all modulator may be miniaturized.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a perspective view of anexample of a Sr Nb O-, single crystal cut for electro-optic measurementwith a representation of the crystallographic axes; and

FIG. 2 is a schematic diagram of a Sr Nb O modulator according to apreferred embodiment of the invention.

DETAILED DESCRIPTION The melting point of Sr Nb O is about l,700 C. Ac-

cording to crystal growth by the Czochralski method, the sinteredmaterial is filled into an iridium crucible made, molten, and thenpulled from the crucible at a rate of about 6mm per hour. With thefloating zone technique, a ceramic rod is formed from the sinteredmaterial in the first place. The ceramic rod and a seed crystal may thenbe placed in an image furnace as disclosed in IEEE TRANSACTIONS; ONMAGNETICS, Vol. MAG-5, No. 3, September 1969,.p. 286, FIG. I,

rotated in opposite directions to each other at 30 RPM and moveddownward in air at a zone speed of 10mm per hour. The transparentcrystal obtained by this process is about 5mm in diameter and 40mm inlength and its density is 5.1 gram per cm.

The single crystal can be prepared even if the molecular ratio of SrO toNb O varies from the stoichiometric composition (2 1) within a range of1.7 l to 2.4

General Properties of Sr Nb O Single Crystal 1. S r Nb-,,O singlecrystal belongs to the rhombic crystal system and the C crystallographicpoint group. This is a biaxial crystal with its optical plane parallelto the c-plane. The crystal has a natural cleavage parallel to theb-plane and a bisectrix of the optical angle normal to the b-plane. Thelattice constants a, b and c satisfy the relation a c b.

2. Sr Nb O single crystal is ferroelectric and its Curie point is nearl,350 C. The direction of spontaneous polarization is along the c-axisand a D-E hysteresis loop can be observed at room temperature. Values ofspontaneous polarization and coercive field are 9 p.C/ :m and 6 kV/cm,respectively.

3. The single crystal is chemically and physically stable and shows nomeasurable deterioration upon exposure to any normally encounteredatmosphere.

4.'Sr Nb O-, single crystal manifests a linear electrooptic effect. Theeffect is largest when a voltage is applied along the c-axis and lightis propagated along the b-axis. In this case, the half-wavefield-distance product is approximately L800 volts at 6,328 angstromunits.

Referring to FIG. 1, the half-wave field-distance product [E.l]A/2 wasmeasured under conditions in which a He-Ne laser beam having a wavelength of 6,328 angstroms propagated through a single domain crystal 1of Sr Nb O having a size: of 2.29mm (aaxis direction) X 0.96mm (baxis) X3.2lmm (c-axis). The values obtained were 1,800 volts when a DC electricfield was applied along the c-axis and a laser beam was propagated alongthe b-axis, 4,300 'volts for an electric field applied along the c-axisand a laser beam along the a-axis, 5,000 volts for an electric fieldapplied along the a-axis and laser light along the b-axis, 32,000 voltsfor an electric field applied along the a-axis and laser light along thec-axis, 135,000 volts for an electric field applied along the b-axis andlaser light along the a-axis, and 750,000 volts for an electric fieldapplied along the b-axis and laser light along the c-axis.

The best value, 1,800 volts, obtained for the strontium niobate crystalis smaller than that of KDP (4,000 volts), LiNbO (2,600 volts) or LiTaO(2,700 volts).

5. The single strontium niobate crystal shows no radiation damage. Afterthe crystal was irradiated by a He-Ne laser beam (6,328 angstrom) of apower of 6 miliwatts and a beam diameter of 70 microns for 2 hours,there was no evidence of radiation damage to the crystal.

6. The relative dielectric constants c], sb and sf of the crystalmeasured at 1 kHz at room temperature are 75, 46 and 43, respectively.The value of 5,? is small enough, and is similar to that of KDP (50),LiNbO (28) and LiTaO (47).

7. The single strontium niobate crystal has a large piezoelectricactivity. The electromechanical coupling coefficient k is. 26 percent,which is larger than that of the LiTaO, single crystal (21 percent).

It will be understood from items (3) to (6) that the Sr Nb O, singlecrystal is the most excellent material for use in electro-opticmodulators that has thus far been proposedJts figure of merit 1/(e.[E.l]A/2) is 5.7 times that of KDP, 1.4 times that of LiNbO and 2.5 timesof LiTaO It is noted that Sr Nb O in a ceramic form has been reported bySmolenskii'et al. in Soviet Physics-Doklady l (l956),-300, who onlymeasured its dielectric constant and tan 6. The remarkable utility of SrNb O,

single domain single crystal particularly in the field of light,however, discovered for the first time by the present invention.

EXAMPLE ya conventional voltage source 14. The crystal 11 is placedbetween a pair of crossed polarizers 15 and 17 which have theirpolarization directions respectively oriented at +45 and 45 with respectto the c-axis of the crystal l1.'A light beam 18 such as that from aHe-Ne laser is polarized by the polarizer 15, focused on the crystal ll,propagated through the crystal along Therefore, the voltage source 14can be realized by a conventional transistor circuit.

It is understood that the invention has been described with reference toa preferred embodiment thereof and that variations are to be includedwithin the scope of the invention. Forexample, although the directionsof the light beam and electric field are specifically described as beingalong the b-axis and c-axis, respectively, they may be along the otheraxes as well. Furthermore, frequency or phase modulation of the lightbeam by the modulator is possible, although the embodiment specificallydescribed performs intensity modulation. Furthermore, although referenceis made in the specification and claims to the nominal composition Sr NbO the electro-optic effect is observed over a compositional range withinwhich the molecular ratio of SrO to Nb O varies from L7 l to 2.4 1.Small amounts of additives in the crystal may thus be tolerated, and allsuch variations are to be included within the terminology nominalcomposition."

What is claimed is:

1. A modulator for modulating a coherent beam of electromagnetic wavecomprising an electro-optic crystalline body, means for transmittingsaid coherent beam into said crystalline body, means for impressing anelectric field across said crystalline body in a direction to modify atransmission property of said beam, said crystalline body including asingle domain, single crystal of a chemical composition (SrO) X Nb O inwhich X ranges from 1.7 to 2.4.

2. The modulator as claimed in claim 1, in which said coherent beam istransmitted into said body along the crystallographic b-axis of saidbody, and said electric field is impressed along the crystallographicc-axis of said body.

3. The modulator as claimed in claim 2, in which said body has arectilinear shape elongated along the crystallographic b-axis of saidcrystalline body.

4. The modulator as claimed in claim 1, in which said crystalline bodyhas a chemical composition Sr Nb O,. k =0

2. The modulator as claimed in claim 1, in which said coherent beam istransmitted into said body along the crystallographic b-axis of saidbody, and said electric field is impressed along the crystallographicc-axis of said body.
 3. The modulator as claimed in claim 2, in whichsaid body has a rectilinear shape elongated along the crystallographicb-axis of said crystalline body.
 4. The modulator as claimed in claim 1,in which said crystalline body has a chemical composition Sr2Nb2O7.